Show where traffic leaves from in tun status

README.md

@@ -411,14 +411,24 @@ vpncli tun down
 
 ```
 up through vpncli-ams3-0a910d (203.0.113.10, ams3) for 8m
-interface utun5: 172.19.0.1/30, fdfe:dcba:9876::1/126, mtu 9000, up
+interface utun5: 172.19.0.1/30, fdfe:dcba:9876::1/126, mtu 65535, up
+exit 203.0.113.10 in NL, the server's address
 ```
 
 The interface is found by the tunnel's own address, because its name is the
 system's choice. A sing-box running with no interface carrying that address is
 reported as routing nothing - which is what one started without root looks
 like, and otherwise looks exactly like a tunnel that works.
 
+The exit line is where the internet thinks you are. Status asks Cloudflare's
+trace endpoint, by IP so no DNS lookup goes anywhere, which address and country
+the request arrived from, and checks the address against the server's. A
+process and an interface only show that a tunnel exists; this shows it carrying
+traffic. An exit that is not the server is a tunnel that is up and routing
+nothing, and a lookup that times out is one that cannot reach the internet at
+all. It is the one request status makes, so it takes a round trip rather than
+being instant.
+
 With no id it takes the most recently configured server, which after a
 provision or a rotation is the one you meant.
 
@@ -533,7 +543,7 @@ vpncli server rotate 3      replace one with a fresh server
 vpncli server destroy 3     delete one and forget it
 
 vpncli tun up 3             route this machine through a server
-vpncli tun status           whether the tunnel is up, since when, and on which interface
+vpncli tun status           whether the tunnel is up, on which interface, and the exit country
 vpncli tun down             stop it
 
 vpncli sync                 reconcile local state against the provider

internal/cli/tun.go

@@ -104,14 +104,17 @@ func newTunStatusCommand() *cobra.Command {
 		Use:   "status",
 		Short: "Say whether the tunnel is up",
 		Long: `Report whether a tunnel is running, through which server, and since when,
-and the network interface it created: its name, addresses, MTU and whether it
-is up.
-
-It reads local state, the process list and the interface table, so it is
-instant and works offline. A sing-box with no interface is reported as routing
-nothing, which is what one started without root does. It does not check that
-traffic is actually flowing - for that, ask something on the internet where it
-thinks you are.`,
+the network interface it created, and where traffic actually leaves from.
+
+The interface line gives its name, addresses, MTU and whether it is up. A
+sing-box with no interface is reported as routing nothing, which is what one
+started without root does.
+
+The exit line asks Cloudflare (` + exitTraceURL + `) which address
+and country a request arrives from, and checks it against the server's. That
+is the one request status makes, and it goes through the tunnel like everything
+else; an exit that is not the server means the tunnel is up and carrying
+nothing.`,
 		Args: cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			t, err := newTunnel()
@@ -222,24 +225,30 @@ func runTunStatus(ctx context.Context, out io.Writer, t *tunnel) error {
 		return err
 	}
 
-	if err := printTunServer(ctx, out, t, running); err != nil {
+	serverIP, err := printTunServer(ctx, out, t, running)
+	if err != nil {
+		return err
+	}
+	if err := printTunInterface(out, t); err != nil {
 		return err
 	}
-	return printTunInterface(out, t)
+	printTunExit(ctx, out, t, serverIP)
+	return nil
 }
 
-// printTunServer says which server the tunnel goes through, and since when.
-func printTunServer(ctx context.Context, out io.Writer, t *tunnel, running record) error {
+// printTunServer says which server the tunnel goes through, and since when,
+// and returns its address, or nothing when that is not known.
+func printTunServer(ctx context.Context, out io.Writer, t *tunnel, running record) (string, error) {
 	if running.Server == 0 {
 		// Running, but not started by this command - so there is nothing to
 		// say about which server or since when.
 		fmt.Fprintf(out, "up, from a tunnel this command did not start (%s)\n", t.configPath())
-		return nil
+		return "", nil
 	}
 
 	store, err := openStore()
 	if err != nil {
-		return err
+		return "", err
 	}
 	defer store.Close()
 
@@ -249,12 +258,42 @@ func printTunServer(ctx context.Context, out io.Writer, t *tunnel, running recor
 		// is what destroying one from another terminal looks like.
 		fmt.Fprintf(out, "up through server %d, which is no longer in local state, for %s\n",
 			running.Server, took(time.Since(running.Started)))
-		return nil
+		return "", nil
 	}
 
 	fmt.Fprintf(out, "up through %s (%s, %s) for %s\n",
 		srv.Name, srv.IPv4, srv.Region, took(time.Since(running.Started)))
-	return nil
+	return srv.IPv4, nil
+}
+
+// printTunExit says where the internet thinks this machine is. It is the one
+// line that shows traffic actually going through the tunnel rather than a
+// process and an interface that merely exist: an exit address that is not the
+// server's is a tunnel that is up and carrying nothing.
+//
+// A lookup that fails is reported rather than returned. The rest of the status
+// is still true, and a tunnel that cannot reach the internet is worth seeing
+// next to it, not instead of it.
+func printTunExit(ctx context.Context, out io.Writer, t *tunnel, serverIP string) {
+	e, err := t.run.Locate(ctx)
+	if err != nil {
+		fmt.Fprintf(out, "exit unknown: %v\n", err)
+		return
+	}
+
+	where := e.IP
+	if e.Country != "" {
+		where += " in " + e.Country
+	}
+	switch {
+	case serverIP == "":
+		fmt.Fprintf(out, "exit %s\n", where)
+	case e.IP == serverIP:
+		fmt.Fprintf(out, "exit %s, the server's address\n", where)
+	default:
+		fmt.Fprintf(out, "exit %s, which is not the server (%s): traffic is not going through the tunnel\n",
+			where, serverIP)
+	}
 }
 
 // printTunInterface describes the interface sing-box created. A running

internal/cli/tun_test.go

@@ -36,12 +36,17 @@ type fakeRunner struct {
 	// being none.
 	link    link
 	linkErr error
+
+	// exit is where traffic arrives from; exitErr is the lookup failing.
+	exit    exit
+	exitErr error
 }
 
 func newFakeRunner() *fakeRunner {
 	return &fakeRunner{
 		version: "sing-box version 1.14.0\n\nEnvironment: go1.26.7 darwin/arm64\n",
 		link:    link{Name: "utun9", MTU: 9000, Up: true, Addrs: []string{"172.19.0.1/30"}},
+		exit:    exit{IP: "203.0.113.10", Country: "DE"},
 	}
 }
 
@@ -72,6 +77,8 @@ func (f *fakeRunner) Start(_ context.Context, _ io.Writer, name string, args ...
 	return nil
 }
 
+func (f *fakeRunner) Locate(context.Context) (exit, error) { return f.exit, f.exitErr }
+
 func (f *fakeRunner) Interface(netip.Addr) (link, error) { return f.link, f.linkErr }
 
 func (f *fakeRunner) Matching(context.Context, string) ([]int, error) { return f.live, nil }
@@ -644,3 +651,91 @@ func TestSystemInterfaceFindsAnAddress(t *testing.T) {
 		t.Errorf("err = %v, want ErrNoInterface for an address nothing has", err)
 	}
 }
+
+func tunStatus(t *testing.T, tn *tunnel) string {
+	t.Helper()
+
+	var out bytes.Buffer
+	if err := runTunStatus(context.Background(), &out, tn); err != nil {
+		t.Fatalf("tun status: %v", err)
+	}
+	return out.String()
+}
+
+func TestTunStatusSaysWhereTrafficLeaves(t *testing.T) {
+	tn, _ := tunneling(t)
+	if _, err := up(t, tn, 1, true); err != nil {
+		t.Fatalf("tun up: %v", err)
+	}
+
+	want := "exit 203.0.113.10 in DE, the server's address"
+	if got := tunStatus(t, tn); !strings.Contains(got, want) {
+		t.Errorf("status = %q, want it to say %q", got, want)
+	}
+}
+
+// Up, with an interface, and leaving from somewhere else: the tunnel is
+// carrying nothing, and only asking the outside can tell.
+func TestTunStatusCatchesTrafficNotInTheTunnel(t *testing.T) {
+	tn, f := tunneling(t)
+	if _, err := up(t, tn, 1, true); err != nil {
+		t.Fatalf("tun up: %v", err)
+	}
+	f.exit = exit{IP: "198.51.100.7", Country: "US"}
+
+	got := tunStatus(t, tn)
+	for _, want := range []string{"198.51.100.7 in US", "not going through the tunnel"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("status = %q, want it to mention %q", got, want)
+		}
+	}
+}
+
+// A failed lookup is part of the status, not the end of it.
+func TestTunStatusWhenTheExitCannotBeFound(t *testing.T) {
+	tn, f := tunneling(t)
+	if _, err := up(t, tn, 1, true); err != nil {
+		t.Fatalf("tun up: %v", err)
+	}
+	f.exitErr = errors.New("context deadline exceeded")
+
+	got := tunStatus(t, tn)
+	for _, want := range []string{"up through", "interface utun9", "exit unknown: context deadline exceeded"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("status = %q, want it to mention %q", got, want)
+		}
+	}
+}
+
+// A tunnel started by hand has no server to compare against, so the exit is
+// reported as it is.
+func TestTunStatusExitWithoutAServer(t *testing.T) {
+	tn, f := tunneling(t)
+	f.live = []int{9001}
+
+	got := tunStatus(t, tn)
+	if !strings.Contains(got, "exit 203.0.113.10 in DE\n") {
+		t.Errorf("status = %q, want the exit with nothing to compare it to", got)
+	}
+}
+
+func TestParseTrace(t *testing.T) {
+	trace := "fl=522f106\nh=1.1.1.1\nip=209.38.105.249\nts=1790991942.000\ncolo=AMS\nloc=NL\ntls=TLSv1.3\n"
+
+	got, err := parseTrace(trace)
+	if err != nil {
+		t.Fatalf("parseTrace: %v", err)
+	}
+	if want := (exit{IP: "209.38.105.249", Country: "NL"}); got != want {
+		t.Errorf("parseTrace = %+v, want %+v", got, want)
+	}
+
+	got, err = parseTrace("ip=192.0.2.1\nloc=XX\n")
+	if err != nil || got.Country != "" {
+		t.Errorf("parseTrace with loc=XX = %+v, %v, want no country", got, err)
+	}
+
+	if _, err := parseTrace("<html>captive portal</html>"); err == nil {
+		t.Error("parseTrace accepted a page with no address in it")
+	}
+}

internal/cli/tunnel.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"net/http"
 	"net/netip"
 	"os"
 	"os/exec"
@@ -65,6 +66,26 @@ type runner interface {
 	// Interface finds the network interface carrying addr, or reports
 	// ErrNoInterface.
 	Interface(addr netip.Addr) (link, error)
+	// Locate asks the internet where this machine's traffic arrives from.
+	Locate(ctx context.Context) (exit, error)
+}
+
+// exitTraceURL is asked where traffic leaves from. Cloudflare's trace answers
+// with the address a request arrived from and the country that address is in,
+// needs no account, and is reached by IP: there is no lookup to leak, and the
+// answer is the IPv4 address, which is the one the server is recorded by.
+const exitTraceURL = "https://1.1.1.1/cdn-cgi/trace"
+
+// exitTimeout bounds the lookup. Through a tunnel to another continent a
+// round trip is a quarter of a second, so this only runs out on one that is
+// not carrying anything.
+const exitTimeout = 5 * time.Second
+
+// exit is where traffic arrives from, as the internet sees it.
+type exit struct {
+	IP string
+	// Country is the ISO 3166 code, "NL", or empty when it is not known.
+	Country string
 }
 
 // ErrNoInterface is returned when no interface carries the tunnel's address.
@@ -304,6 +325,54 @@ func (system) Interface(addr netip.Addr) (link, error) {
 	return link{}, ErrNoInterface
 }
 
+func (system) Locate(ctx context.Context) (exit, error) {
+	ctx, cancel := context.WithTimeout(ctx, exitTimeout)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, exitTraceURL, nil)
+	if err != nil {
+		return exit{}, err
+	}
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return exit{}, fmt.Errorf("asking %s: %w", exitTraceURL, err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return exit{}, fmt.Errorf("asking %s: %s", exitTraceURL, resp.Status)
+	}
+	body, err := io.ReadAll(io.LimitReader(resp.Body, 4096))
+	if err != nil {
+		return exit{}, fmt.Errorf("reading %s: %w", exitTraceURL, err)
+	}
+	return parseTrace(string(body))
+}
+
+// parseTrace reads Cloudflare's trace, which is key=value lines.
+func parseTrace(body string) (exit, error) {
+	var e exit
+	for line := range strings.SplitSeq(body, "\n") {
+		key, value, ok := strings.Cut(strings.TrimSpace(line), "=")
+		if !ok {
+			continue
+		}
+		switch key {
+		case "ip":
+			e.IP = value
+		case "loc":
+			// XX is Cloudflare for not knowing.
+			if value != "XX" {
+				e.Country = value
+			}
+		}
+	}
+	if e.IP == "" {
+		return exit{}, fmt.Errorf("%s did not say where the request came from", exitTraceURL)
+	}
+	return e, nil
+}
+
 // usable checks that the client is installed and new enough to read what this
 // generates.
 //